Way back when cyber used to be all about your perimeter. Organizations would put in a firewall and other tools in an effort to keep the bad guys out of the network. However, there is one fatal flaw in that methodology- what do you do when the attacker has gotten past your perimeter and is on your network? How do you stop the attack? How do you even know the attack is occuring?
In the modern world, its not if but when you will have a cyber issue. The best defense not one defense at all. It is combining different techniques into a layered defense. The effect is similear to peeling back an onion. As an attacker gets through one layer there are more layers of protection. It is also important to note that not all the layers should be about prevention. Some of the layers needs to be about understanding when something is starting to happen so you can take action- i.e. logging and montioring. None of the layers are an ultimate barrier, but combined they reduce your risks.
For an example, an lets look at a house. The perimeter might have a fence that an attacker would have to get passed. If the attacker made it past the fence, perhaps there is a dog in the yard the attacker would have to make it passed. If they get by the dog, there might be security cameras on the house. The might be locks on the doors and windows. If an attacker got by all that there might be an alarm system and internal house cameras. A house would not generally have only one line of defense. Neither should your cybersecurity. You might want a firewall at the perimeter. Locks on the doors and windows and an alarm system for phyiscal security. An account management process governing who is given accounts and when they are removed are a must as are employee background checks. Computers should have their hard drives encrypted. The list goes on. Cyber is not a one and done. Cyber needs to be layered like an onion to be effective.